I'd use masking tape. So far, for every single signature quilt I've made, someone has signed so part of their name goes into the seam allowance.
I'm not sure why some people have such a strong desire to sign right on the very edge, but plenty do.
Even when I've asked people to sign at least 1" from the edge, someone will start out at 1" and then do a looping letter with a tail and end up right off the edge. I suppose even with tape, someone is going to sign so part of their name is on the tape.
The main problem I've had with kids is that they decide they've made a mistake and then they scribble it out and start over.